Current 3GPP specifications provide various scenarios to moving a mobile terminal or user equipment, UE, from one radio access network, RAN, to another RAN. For example 3GPP TS 23.401, version 12.2.0, in the following being referred to as TS 23.401, discloses in section 5.5.2 different inter radio access technologies, RAT, handover scenarios RAT, e.g. from a circuit switched RAN (e.g. GERAN or UTRAN) to a packet switched RAN (e.g. a so-called evolved UMTS Terrestrial Radio Access Network, E-UTRAN). In case of multiple available RANs, such moving may be controlled e.g. by prioritizing one radio access type or technology over another one, e.g. prioritizing E-UTRAN over GERAN/UTRAN.
According to current 3GPP specifications, the UE is moved to E-UTRAN when receiving the Clear Command from the MSC, which will also release the RR connection. The MSC may send the Clear Command if all CS services are finished.
E.g. in order to allow a fast return to LTE after a so-called circuit switched fallback, CSFB, as being specified in 3GPP TS 23.272 version 12.0.0, e.g. the GERAN (i.e. the base station controller, BSC) notifies the UE about the LTE frequency at channel release, i.e. when BSC has received a so-called Clear Command from the MSC. Both the BSC and the UE will release its radio resource, RR, connection at reception of the channel release.
The RAN currently serving the UE may decide to move the UE to the other RAN based on rather static instructions, e.g. based on priority instructions provided by the core network (as currently specified for SGSN and MME). However, in some cases a prompt execution of such instructions may not be optimum. E.g. in a case wherein the core network and the UE status are not synchronized, and an immediate movement of the UE from the current RAN to the other RAN may prevent from completing useful or required signaling.
As an example, in a case wherein the UE has performed CSFB for an emergency call and is camping on a mobile switching center, MSC, which does not have subscriber data associated to the UE (to be provided by the corresponding visitor location register, VLR) the UE does not perform a location area update, LAU, prior to establishing the emergency call, but the MSC might have performed an “Update Location” procedure using IMSI. Hence a LAU would be required after the emergency call is completed, but if the UE is moved to E-UTRAN in the meantime, the LAU cannot be completed.
In another example, The MSC may have detected than the Temporary Mobile Subscriber Identity, TMSI, should be reallocated during a call, but the completing the TMSI reallocation procedure may not be possible as the UE is moved to E-UTRAN in the meantime.